This invention generally relates to apparatus for controlling the exposure area of a film; and more specifically, to an apparatus that is particularly well suited for controlling the area of an x-ray film that is exposed when x-rays are used to produce a topograph of crystal wafers. A preferred embodiment of the invention disclosed herein in detail relates to apparatus for seamless topographic imaging large single crystal wafers, and in particular, for controlling the area of an x-ray film that is exposed when x-rays are scanned across a single crystal wafer to produce an x-ray topograph of the wafer.
Single crystal wafers, which are commonly used to make semiconductors, are normally inspected after being formed. These wafers may be inspected by using x-rays to produce images of the top surfaces of the wafers, and to do this, x-rays are reflected off that top surface and onto x-ray film. With one specific procedure, an x-ray beam is reflected off an area of the top surface of the wafer and onto an area of the x-ray film. This process is repeated across the top surface of the wafer, to generate an image of the entire top surface of the wafer.
With this prior art procedure, it is often difficult to produce clear, sharp images on the x-ray film. In the former procedure, the x-rays are diffracted from the surface area of the wafer at various angles depending on the wavelength of the x-rays. Multiple images generated on the x-ray film may overlap, obscuring the information contained on these images. In addition, if the x-rays initially illuminate a large area of the wafer, x-ray fluorescence will cause the x-ray film to fog and degrade the topographic images.